INORGANIC GEOCHEMISTRY

Inorganic chemical analyses were conducted on seven interstitial water samples from Hole 1147A squeezed from whole-round samples at a frequency of one per core in the first five cores and one every second core thereafter. Analytical methods are detailed in "Inorganic Geochemistry" in the "Explanatory Notes" chapter. The concentrations of dissolved interstitial constituents are presented in Table T13, and the profiles with depth are shown in Figure F13. This short interval was drilled to recover a recent sedimentary section thought to be missing at Site 1148, and as such it yields an abbreviated picture of sediment water interactions. The primary interstitial water variations observed at this site are those mediated by sulfate reduction of organic matter. Longer term reactions, such as clay alteration and diagenesis of biogenic sediments, were not well expressed in the nine cores recovered at this site.

Chloride (Cl^-) concentrations in interstitial waters at Site 1147 increase from 545 mM near the surface to 559 mM at 19 mcd and are relatively constant below this depth (Fig. F13A; Table T13). Interstitial water salinities decrease from 35 to 34 between 3 and 10 mcd and then are constant below this depth (Fig. F13B; Table T13).

Sulfate decreases from 26.6 mM in the first core to 21.2 mM by the second core and is relatively constant below this depth, 17.3 ± 2.6 mM (Fig. F13C; Table T13), indicating that sulfate reduction is incomplete at this site. Ammonium (NH₄⁺) increases between the first and second core as well and then increases slightly downhole (Fig. F13D; Table T13). Dissolved phosphate (HPO₄^2-) concentrations increase to a maximum of 47.7 mM in the zone of sulfate reduction and then decrease rapidly to reach near-zero values by 84 mcd (Fig. F13E; Table T13). Alkalinity increases to a maximum of 11.8 mM by the third core, then decreases downhole (Fig. F13F; Table T13). A minimum in pH is centered at the depth of the maximum in NH₄⁺, HPO₄^2-, and alkalinity (Table T13). Incomplete sulfate reduction and relatively constant or decreasing concentrations of methanogenic products NH₄⁺, HPO₄^2-, and alkalinity below the zone of sulfate reduction are consistent with the low methane values at this site, indicating that methanogenesis is not an active process in these sediments.

Magnesium (Mg²⁺) concentrations decrease with depth from 52.9 mM at the top to a minimum of ~46.1 mM at the bottom of the hole (Fig. F13G; Table T13). Dissolved calcium concentrations (Ca²⁺) decrease rapidly in the zone of sulfate reduction from a maximum of 11.1 mM in the first core to 8.0 mM by the third core, presumably reflecting the removal of Ca²⁺ through inorganic calcite precipitation, and then decrease very slightly toward the base of the hole (Fig. F13H; Table T13). Dissolved potassium (K⁺) concentrations decrease downhole from ~11.7 mM near the surface to 8.9 mM at the base of the hole (Fig. F13I; Table T13). Changes in all three of these elements are very similar to the corresponding profiles at Site 1146 over the same ~80-m depth interval, suggesting that similar processes of clay and volcanic ash alteration are at work deeper at this site.

Dissolved silica (H₄SiO₄) concentrations increase to a maximum of 769 mM at 32 mcd and then decrease below this depth (Fig. F13J; Table T13). Lithium (Li⁺) decreases between the first and second core to reach a minimum of 24 mM, increases to a maximum of 33 mM at 84 mcd, and then decreases downhole (Fig. F13K; Table T13). Dissolved strontium concentrations (Sr²⁺) increase downhole from a minimum at the surface of 86 mM to a maximum of 153 mM at the base of the hole (Fig. F13L; Table T13). Changes in Li and Sr with depth at Site 1147 are relatively small, reflecting the small changes in diagenesis of biogenic sediments in the upper sediments.